Rotary pump and motor hydraulic drive having a substantially constant output speed



2,630,681 DRIVE HAVING PUT SPEED 5 Sheets-Sheet l INVENTOR.

ATTORNEY WALTER FERRIS BY WW FERRIS HYDRAULIC NSTANT OUT W. ROTARY PUMP AND MOTOR A SUBSTAN'IIALLY CO FIG. 1

March 10, 1953 Filed Nov. 4, 1950 I 1 m 2 XM 7 a W 2 w .w V/ww i W W VW Q 1 2 w7/// PW Til WW //A MO-TOR March 10, 1953 FERRIS 2,630,681

W. ROTARY PUMP AND MOTOR HYDRAULIC DRIVE HAVING A SUBSTANTIALLY CONSTANT OUTPUT SPEED Filed. Nov. 4, 1950 3 Sheets-Sheet 2 6 1 FIG. 2 l2 1?.

PUMP ENGINE Y umvs JNVENTOR. WALTER FERRIS ATTORNEY March 10, 1953 w. FERRIS ROTARY PUMP AND MOTOR HYDRAULIC DRIVE: HAVING A SUBSTAN'IIALLY CONSTANT OUTPUT SPEED Filed Nov. 4, 195 0 3 Sheets-Sheet 3 FIG. 5 2-] INVENTOR. WALTER FERRIS ATTORNEY Patented Mar. 10, 1953 A ROTARY PUMP AND MOTOR HYDRAULIC DRIVE HAVING A SUBSTANTIALLY CON SiTANT OUTPUT SPEED Walter Ferris, Milwaukee, Wis., assignor to The Oilgear Company, Milwaukee, Wis., a corporation of Wisconsin Application November 4, 1950, Serial No. 194,160

12 Claims.

This invention relates to hydraulic drives of the type which includes a motor for driving a load, a pump for supplying liquid to the motor to em ergize the same and means for maintaining the speed of the motor within a narrow speed range regardless of wide variations in the speed at which the pump is driven.

An object of the invention is to provide a hydraulic drive of the above type which is particularly adapted for driving refrigerating apparatus, which is carried by a vehicle, from the power plant which drives the vehicle.

Another object is to provide a drive of the above type in which the pump and the motor may be of the sliding vane type.

Another object is to provide a drive of the above type which is light in weight, compact and economical to manufacture.

Other objects and advantages will appear from the description hereinafter given of hydraulic drives in which the invention is embodied.

According to the invention in its principal aspect, the displacement of the pump is varied in response to variations in the drop in pressure across an orifice through which liquid is discharged by an auxiliary pump which is driven in unison with the motor.

The invention is exemplified by the drives shown schematically in the accompanying drawings in which the views are as follows:

1 is a diagram of the principal parts of the hydraulic circuit of a drive in which the invention is embodied and in which the vanes of the motor are urged outward against the vane track by springs.

Fig. 2 is a circuit diagram of a drive similar to that shown in Fig. 1 but in which the motor vanes are moved outward in response to the drop in pressure across an orifice through which liquid flows from the pump to the motor, the motor being represented by a diagram of the main ports and the vane slot ports in its cheek plate arranged in the plane indicated by the line 2-2 of Fig. 5.

Fig. 3 is a circuit diagram of a drive similar to that shown in Fig. 2 but in which the motor vanes are moved outwardand the displacement of the pump is varied in response to the drop in pressure across a single orifice.

Fig. 4 is a transverse section through ,a hydraulic motor which may be employed in the drives shown in Figs, 2 and 3, the view being taken on the line 44 of Fig. 5.

Fig. 5 is in part a top plan view of the motor shown in Fig. 4 and in part a horizontal section taken on the line 5-5 of Fig. 4,

For the purpose of illustration, the invention will be explained as being embodied in a drive in which the motor is employed to drive a refrigerating apparatus which is carried by a motor truck and the pump is driven by the engine of the truck but it is to be understood that the invention is not limited to such use as a drive embodying the invention may be employed to drive a load and to maintain the speeds of the load within a narrow speed range regardless of variations in the speed of the pump driving means or regardless of variations in the load on the motor which tend to cause variations in the motor speed.

Figure 1 The drive includes a variable displacement pump 1- which is driven by the engine of a refrigerator truck as indicated in Fig. 3, a hydraulic motor 2 which is energized by liquid from pump 1 and drives the truck refrigerating apparatus (not shown), a small pilot pump 3 which is driven in unison with motor 2 and discharges through an orifice 4, and a pressure responsive control 5 which effects regulation of the displacement of pump l in response to variations in the drop in pressure across orifice 4.

Any suitable type of variable pump may be employed but, in order to keep the weight and overall dimensions of the pump at minimum, pump l preferably is of the sliding vane type shown in Patent No. 2,238,062. However, the pump used in practice is provided with a telescoping vane track such as that shown in Patent No. 2,538,193 instead of a track having flexible sections as shown in Patent No. 2,238,062. Pump I has its outlet connected to the inlet of motor 2 by a channel 6 and .its inlet connected to the outlet of motor 2 by a channel 1 which is also connected to a tank (not shown) from which pump 1 may draw liquid to make up for leakage losses.

The displacement of pump I is determined by the positions of two diametrically opposed bridges 8 and 9 which form parts of the vane track of the pump. Pump displacement is maximum when both bridges are in their outermost positions, pump displacement is reduced when one or both bridges are moved inward, and pump displacement is zero when both bridges are in their innermost positions. Each of bridges 8 and 9 is fixed for movement with a piston In which is urged outward by a spring I I and is fitted in a cylinder I2 attached to the pump casing. When the pump is operating, bridges 8 and 9 are also urged outward by pump pressure. Springs l I normally hold bridges 8 and 9 in their outermost or maximum displacement positions.

The arrangement is such that, when liquid under pressure is supplied to cylinders I2, pistons It] will move bridges 8 and E3 inward to reduce the displacement of pump I, and than when liquid is permitted to escape from cylinders I2, springs II will move bridges and outward to increase the displacement or" pump I.

Motor 2 may be of any suitable type but, in order to keep its weight and dimensions at minimum, the motor used in practice is of the sliding vane type and is similar to the motor shown in Figs. 4 and 5 except that its vanes are urged outward against the vane track by springs as shown in Patent No. 2,357,333. Pilot pump 3 has been shown separate from motor 2 but in practice it is arranged within the casing thereof and driven in unison therewith as shown in Fig. 5. Also, orifice 4 is arranged Within the casing of motor 2. Pump 3 has its inlet connected to discharge channel I and its outlet connected by channel I3 to the inlet of orifice 4 the outlet of which is connected to channel I so that when driven pump 3 will draw liquid from channel I and discharge it through orifice 4 back into channel 1.

Control 5 includes a body It having formed therein a small axial bore I5, a large chamber It at one end of bore I5 and a port I'I formed in the wall of bore I5 and connected to both of cylinders I2 by a channel It. A valve IE3 is fitted in bore I5 to control communication between channel I8 and two channels and 2! which communicate with bore I5 at opposite sides of port I? and are connected to channels 6 and 7 respectively.

.Valve I9 is urged toward the left by a spring 22 arranged in a cap 23 which is fastened to the end of body It and is provided in its outer end with a screw 24 for adjusting the resistance of Spring 22. moved toward the right in response to an ncrease in the drop in pressure across orifice 4 such as by means of a piston having one of its ends in contact with the end of valve It and its opposite ends subjected to the pressures prevailing at opposite sides of orifice 4. Preferably however, the end of valve it engages the closed end of a bellows type actuator or sylphon 25 which is arranged in chamber It and has its interior connected by a channel 26 to channel I 3.

The interior of cap 23 is connected to chamber It by a channel 21, so that both ends of valve I9 are subjected to the same pressure, and chamber It is connected by a channel 28 to the same part of the circuit to which the outlet of orifice 4 is connected so that, when pilot pump 3 is running, the force exerted by sylphon 25 upon valve I9 is proportional to the drop in pressure across orifice 4. In the present instance, channel 28 and the outlet of orifice i are connected to channel I in which the pressure is substantially zero so that the forc exerted by sylphon 25 upon valve I9 is substantially proportional to the pressure created by pump 3 in discharging liquid through orifice 4.

Spring 22 and sylphon 25 are so proportioned and spring 22 is initially so adjusted by turning screw 24 that the pressure in sylphon Valve I9 is adapted to be 25 need exceed the pressure in chamber is by in sylphon 25. Therefore, spring 22 will hol valve I9 to the left of its neutral position so that channel I8 is open to channel 2I and springs II will hold pump I at its maximum displacement because there is no pressure in cylinders It.

When pump I is started, it will deliver liquid through channel 6 to motor 2 and cause motor 2 to operate and to return liquid to pump I through channel I. If pump I is driven from the engine of a refrigerator truck, as indicated in Fig. 3, and if motor 2 is employed to drive the refrigerating apparatus of the truck, the truck engine is so adjusted that when running at its idling speed it will drive pump I fast enough to enable pump I to discharge liquid at the rate required to drive motor 2 at the desired speed. that is, the speed at which motor 2 will drive the refrigerating apparatus at the correct speed.

Since pilot pump 3 is driven in unison with motor 2, it will draw liquid from channel '3 and discharge it through channel I3 and orifice back into channel I and the pressur created by pump 3 in forcing liquid through orifice 4 will extend through channel 26 into sylphon 25. Orifice 4 is so proportioned that, when motor 2 is running at the desired speed, pump 3 will create the pressure, such as 25 p. s. 1., required to enable sylphon 25 to move valve I9 into its neutral position against the resistance of spring 22.

Then when the truck engine is accelerated and drives pump I at a faster speed, pump I will momentarily discharge liquid at a faster rate and thereby momentarily increase the speed of motor 2 and pump 3. Increasing its speed causes pump 3 to force liquid through orifice a at a faster rate and, since the drop in pressure across an orifice, is substantially proportional to the square of the velocity of the liquid flowing therethrough, the pressure created by pump 3 will increase and enable sylphon 25 to move valve I9 toward the right to partially uncover port I'I. Then high pressure liquid will flow from channel It through channel 20, control 5 and channel I8 into cylinders I2 and cause pistons It to reduce the displacement of pump I and thereby reduce the speed of motor 2 and pump 3 until the pressure drop across orifice 4 is reduced to its initial value, such as 25 p. s. i., at which time motor 2 is again running at the desired speed. Decreasing the pressure drop across orifice 4 to its initial value reduces the force exerted by sylphon 25 and enables spring 22 to return valve IE! to its neutral position where it will remain until the speed of motor 2 again varies from the desired speed.

When the truck engine is decelerated, pump I will be driven at a slower speed and will momentarily discharge liquid at a slower rate, thereby momentarily decreasing the speed of motor 2 and pump 3. Decreasing the speed of pump 3 causes the pressure in slyphon 25 to drop and enable spring 22 to move valve I9 toward the left to partially uncover port I'I. Then liquid can flow from cylinders I2 through channel I8, control 5 and channel 2| into return channel I, thereby permitting springs II to increase the displacement of pump I which will increase the speed of motor 2 and pump 3 until the. pressure drop across orifice 4 is increased to its initial value, such as 25 p. s. i., at which time motor 2 is again running at the desired speed. 7, Increasing the pressure drop across orifice 4 to its initial value, enables sylphon '25 to, return valve. I 9 to its initial position where it will remain until the speed of motor Zvaries again from the desired speed.

Each time the speed of pump I varies and causes a variation in the speed of motori, the control will function in the above described manner to vary the displacement of pump I just enough to effect correction of the variation in motor speed. If the load on motor 2 should vary and cause a variation in the speed of motor 2, there would be a corresponding variation in the speed of pilot pump 3 and, consequently, a variation in the drop in pressure across orifice 4 which would cause control 5 to eifect adjustment of pump I to correct the variation in the speed of motor 2 in the same manner as when the variation in motor speed is due to a variation in the speed of pump I. The control will thus maintain the speed of motor 2 within a narrow speed range regardless of whether the speed of pump I is constant or variable and regardless of whether the load on motor 2 is constant or variable.

Figures 4 and 5 The motor shown in these figures, which has been indicated in its entirety by the reference numeral 2, includes a spacer ring 3i having an approximately elliptical vane track 32 formed upon its inner periphery, a rotor 33 arranged within spacer ring 3] and provided with a plurality of radial vane slots 34. and a vane 35 slidably fitted in each slot and having its outer end held against vane track 32. when the motor is operating as will presently be explained.

Spacer ring 3I and rotor 33 are arranged between two identical cheek plates 33 each of which is provided with two diametrically opposed inlet ports 31 and two outlet ports 33 which are spaced 90 from ports 31. Each cheek plate 36 also has a vane slot port 39 formed therein radially inward from each inlet port 31 and a vane slot port 40 formed therein radially inward from each outlet port 38 as indicated in Fig. 2 in which motor 2 has been represented by a face view of one of the cheek plates 36, the View being taken on the line 22 of Fig. 5 but drawn to a smaller scale and with the outer peripheral portion of the cheek plate broken away.

Main ports 31 and 38 extend completely through each cheek plate but the vane slot ports 39 and. 4B are formed only in the inner faces of the cheek plates. Each vane slot port is sup plied with liquid at a pressure higher than the pressure in inlet port 31 as will presently be explained. Each vane slot port 43 is connected to the radially outward outlet port 38 by a groove 4I formed in the outer face of the cheek plate. Vane slot ports 39 and 40 are on the same radius as the inner ends of vane slots 34 so that each vane slot registers with ports 39 and 43 alternately as rotor 3 rotates.

Vane track 32 differs from the vane track of pump I in that it is rigid and has a fixed length while the vane track of pump I is extensible and has a variable length. As previously explained, vane track 32 is approximately elliptical. The portion of vane track 32 between the adjacent ends of ports 3! and 38 at the smallest diameter of the ellipse constitute sealing bridges 42 and the portion of vane track 32 between the adjacent ends of ports 31 and 38 at the greatest diameter of the ellipse constitute working bridges 43 which correspond to the bridges 8 and 9 of pump I but are stationary.

When the motor is operated, at least one vane 35 is always in contact with each of bridges 42 6. and 43 and is positively held thereagainst due to its inner edge being subjected to the pressure in a vane slot port 39. The vanes in contact with the bridges provide seals between ports 3? and 38 so that no liquid can flow from a port 31 into a port 33 without rotating rotor 33.

One of the two cheek plates 35 has its outer face in engagement with a front head 44 having an inlet 45 and an outlet 46 formed therein. Front head 44 contains passages (not shown) which connect inlet 45 to both inlet ports 31 and other passages (not shown) which connect outlet 45 to outlet ports 38. Rotor 33 is splined upon a drive shaft 41 which is rotatably sup ported within front head 44 by means of bearings not shown.

The outer face of the other cheek plate 35 engages one face of a plate 48 the other face of which engages a rear head 49 having formed therein a pump chamber 53 the front end of which is closed by plate 48. Spacer ring 3i, cheek plates 35, plate 48 and heads 44 and 43 are rigidly secured together by a plurality of stud bolts 5|.

Pump chamber 50 has closely fitted therein two intermeshing gears 52 and .53 which constitute the pumping elements of a pilot pump 3 and which are journaled, respectively, upon two stub shafts 54 and 55 fixed in rear head 49. Gear 52 drives gear 53 and it is driven in unison with motor 2 through a coupling 53 which connects shaft 41 to the hub 51 of gear 52.

The arrangement is such that, when motive liquid is supplied to inlet 45, it will flow through ports 31 into the spaces between rotor 33 and vane track 32 and will act upon the outer portions of the vanes 35 in contact with bridges 43, thereby causing rotor 33 to rotate in a clockwise direction in respect to Fig. 4 as indicated by the arrow. Rotor 33 will drive shaft 4'! which will drive pilot pump 3.

Figure 2 The drive shown in this figure is substantially the same as the drive shown in Fig. 1 except that the motor vanes are urged outward by fluid pressure instead of by springs. Consequently, like parts have been indicated by like reference numerals and further description thereof is not necessary. The drive includes a motor 2* which may be the same as the motor shown in Figs. 4 and 5. In order to illustrate how the motor vanes are urged outward by fluid pressure, motor 2 has been represented by a View taken across the face of a cheek plate 36 as indicated by the line 2--2 of Fig. 5. I

Pressure for urging the vanes of motor 2 outward is provided by meansof a channel til through which pump I delivers liquid to the vane slot ports 39 of motor 2 at substantially full pump pressure and by means of an orifice 85 through which pump I delivers liquid. to the inlet ports 3'! of motor 2% and which causes a small drop in pressure, such as 25 p. s. i., between pump l and ports 31 so that the pressure in vane slot ports 39 exceeds the pressure in inlet ports 37 by a small amount such as 25 p. s. i. Orifice H has been shown separate from motor 2 and channel 53 has been shown connected to channel 6 at a point between orifice 5i and pump I but in practice channel and orifice 6| are arranged inside the casing of motor 2. I

Since the pressure in vane slot ports 33 is higher than the pressure in inlet ports 31,it

will be obvious from a comparison of Fig. 4 with the face view of cheek plate 36 shown in Fig. 2 that the vanes in contact with bridges 42 and 43 are held thereagainst by the pressure in vane slot ports 39 and that each vane as it moves from a bridge 42 to the adjacent bridge 43 will be moved outward and held against the vane track by the liquid delivered by pump l to vane slot ports 39. As each vane moves from a bridge 43 to the adjacent bridge 42, it will be moved inward by the vane track and will be hydrostatically balanced due to vane slot ports 40 being connected to outlet ports 38 by grooves 4|.

The drive will function in exactly the same way that the drive shown in Fig. 1 functions. That is, pump I when driven will energize motor 22 and control will function in the previously described manner to maintain the speed of motor 2 within a narrow speed range.

Figure 3 The drive shown in this figure differs primarily from the drive shown in Fig. 2 in that the motor vanes are moved outward and the pump displacement is varied in response to the drop in pressure across the same orifice. Also, in order to simplify the view and to indicate that the shaft d2 of the main pump is connected by a drive 63 to the shaft 34 of a truck engine so that the pump will be driven by the engine, the circuit has been shown as being reversed in respect to the circuit shown in Fig. 2. That is, the pump, the motor and the pump control have been shown turned end for end in respect to Fig. 2.

Since the drive is otherwise the same as the drive shown in Fig. 2, like parts have been indicated by like reference numerals so that only a brief description of the drive is necessary. As shown, the drive includes a main pump 1, a motor 2 a pilot pump 3, an orifice 5, a pump control 5, a channel 6 which connects the outlet of pump I to the inlet of motor 2*, a channel I which connects the outlet of motor A to the inlet of pump 1, a channel l3 which connects pilot pump 3 to orifice 4, a channel It which connects the port I! (Fig. 1) in control 5 to the control cylinders l2 of pump I, a channel 2% which connects high pressure channel 6 to control 5 at one side of port 11, a channel 2i which connects return channel I to control 5 at the other side of port I! and a channel 26 which connects the interior of sylphon 25 (Fig. 1) in control 5 to channel l3, all of which are the same as in the circuit shown in Fig. 2.

The circuit shown in Fig. 3 difiers primarily from the circuit shown in Fig. 2 in that the inlet of pump 3 and the outlet of orifice 4 are connected to channel 6 instead of being connected to channel 1, in that channel 28 is connected to channel 6 instead of to channel '1, because the pressure on the outside of sylphon 25 (Fig. 1) must be substantially the same as the pressure at the outlet of orifice 4, and in that channel 6!] is connected to channel I 3 instead of to high pressure channel 5.

When pilot pump 3 is driven, it will draw liquid from channel 6 and will discharge it through channel 13 and orifice 4 back into channel 6. The resistance of orifice 4 will cause pump 3 to raise the pressure in channel I3 above the pressure in channel 6 and in motor ports 31 by a small amount, such as 25 p. s. i., and that increased pressure will extend through channel as into sylphon 25 and through channel 50 into vane slot ports 39. While the pressure in channel I3 is higher than the pressure in channel 6, the energy consumed by pump 3 is small because it creates only the small difference between the two pressures.

The motor vanes in contact with bridges 42 and 43 and the vanes moving from each bridge 42 to the adjacent bridge 43 are positively held against the vane track by the pressure in vane slot ports 39 and the vanes moving from each bridge 42 to the adjacent bridges 43 are moved outward by liquid supplied by pump 3 through channels l3 and 6D and vane slot ports 39 to the inner ends of the slots containing those vanes.

The pressure within sylphon 25 of the drive shown in Fig. 3 is higher than in the sylphon 25 of either of the drives shown in Figs. 1 and 2 but the difierence between the pressures on the inside of the sylphon and the pressure on the outside thereof is determined by the drop in pressure across orifice 4 the same as in the other two drives. Consequently, a variation in the drop in pressure across orifice 4, due to a variation in the speed of motor 2 and the resultant variation in the speed and rate of discharge of pump 3, will cause control 5 to effect adjustment of pump 1 to correct the variation in motor speed in exactly the same manner as in the drives shown in Figs. 1 and 2.

The hydraulic drive illustrated and described herein. may be modified in various other ways and adapted to various uses without departing from the scope of the invention which is hereby claimed as follows:

1. A hydraulic drive comprising a hydraulic motor, a main pump for supplying liquid to said motor to energize the same and provided with hydraulic servo-motor means for varying its displacement, fluid channels connecting said pump to said motor and forming therewith a hydraulic circuit, a restricted orifice, a pilot pump, a fluid channel connecting said pilot pump to said orifice to enable said pilot pump when driven to discharge liquid through said orifice and thereby cause a drop in pressure across said orifice, means for driving said pilot pump at a speed proportional to the speed of said motor so that a vari ation in the speed of said motor will cause a corresponding variation in the speed and rate of delivery of said pilot pump and a resultant variation in the drop in pressure across said orifice, and means for supplying operating liquid to said displacement varying means including a control which controls the flow of said operating liquid and is responsive to variations in the drop in pressure across said orifice for causing said displacement varying means to vary the displacement of said main pump inversely to variations in the speed of said motor to thereby maintain the speed of said motor within a narrow speed range.

2. A hydraulic drive according to claim 1 including fluid channels which connect said control to said hydraulic circuit and to said servomotor means so that said servo-motor means is adapted to be energized by liquid supplied thereto from said main pump and said control controls the flow of liquid to and from said servo-motor means.

3. A hydraulic drive according to claim 2 in which said motor is of the type having vanes which are moved inward and outward during operation of the motor and which includes fluid channels which connect said pilot pump to said motor at the inner edges of said vanes to cause said vanes to be moved outward by liquid delivered to the inner edges thereof by said pilot pump.

4. A hydraulic drive according to claim 1 in which said motor is of the type having vanes which are moved inward and outward during operation of the motor and which includes fluid channels which connect said pilot pump to said motor at th inner edges of said vanes to cause said vanes to be moved outward by liquid delivered to the inner edges thereof by said pilot pump.

5. A hydraulic drive according to claim 1 in which said control includes a pressure responsive element which has one face thereof subjected to the pressure at the inlet of said orifice and the other face thereof subjected to the pressure at the outlet of said orifice and which effects operation of said control in response to variations in the drop in pressure across said orifice.

6. A hydraulic drive according to claim 5 in which said pressure responsive element comprises a bellows having its interior subjected to the pressure at the inlet of said orifice and its exterior subjected to the pressure at the outlet of said orifice.

7. A hydraulic drive comprising a main pump having hydraulic servo-motor means for varying its displacement, a hydraulic motor energized by liquid delivered thereto by said pump, fluid channels connecting said pump and said motor and forming therewith a hydraulic circuit, a pilot pump for discharging liquid at limited rates, a restricted orifice connected to said pilot pump to resist the discharge of liquid therefrom and thereby cause said pilot pump to create pressure and to Vary said pressure in response to variations in the rate at which it discharges liquid through said orifice, means for driving said pilot pump at a speed proportional to the speed of said motor so that a variation in the speed of said motor will cause a corresponding variation in the speed and rate of delivery of said pilot pump and a resultant variation in the pressure created by said pilot pump, and means for supplying operating liquid from said circuit to said displacement varying means including a control which controls the fiow of said operating liquid and is responsive to variations in the pressure created by said pilot pump for causing said displacement varying means to decrease the displacement of said main pump in response to an increase in said pilot pump pressure and to increase the displacement of said main pump in response to a decrease in said pilot pump pressure.

8. A hydraulic drive according to claim 7 in which said motor is of the type having vanes which are moved inward and outward during operation of the motor and which includes fluid channels which connect said pilot pump to said motor at the inner edges of said vanes to cause said vanes to be moved outward by liquid delivered to the inner edges thereof by said pilot pump.

9. A hydraulic drive according to claim 7 in which said control includes a pressure responsive element which has one face thereof subjected to the pressure at the inlet of said orifice and an opposite face thereof subjected to the pressure at the outlet of said orifice and which effects operation of said control in response to variations in the drop in pressure across said orifice.

10. A hydraulic drive comprising a hydraulic motor, a main pump for supplying liquid to said motor to energize the same and provided with hydraulic servo-motor means for varying its displacement, fiuid channels connecting said pump to said motor and forming therewith a hydraulic circuit, a restricted orifice, a pilot pump, a fluid channel connecting said pilot pump to said orifice to enable said pilot pump when driven to discharge liquid through said orifice and thereby cause a drop in pressure across said orifice, means for driving said pilot pump at a speed proportional to the speed of said motor so that a variation in the speed of said motor will cause a corresponding variation in the speed and rate of delivery of said pilot pump and a resultant vari ation in the drop in pressure across said orifice, means for supplying liquid to said servo-motor means including a valve normally occupying a neutral position and shiftable in one direction or the other to direct liquid to or from said servomotor means to cause the same to decrease or increase the displacement of said main pump, and pressure responsive means associated with said valve and connected to opposite sides of said Orifice for causing said valve to shift in a direction to effect a decrease in the displacement of said main pump in response to an increase in the pressure drop across said orifice or to shift in a direction to effect an increase in the displacement of said main pump in response to a decrease in the pressure drop across said orifice and thereby correct the variation in motor speed which caused the variation in the pressure drop across said orifice.

11. A hydraulic drive according to claim 10 in which said pressure responsive element comprises a bellows having its interior subjected to the pressure at the inlet of said orifice and its exterior subjected to the pressure at the outlet of said orifice.

12. A hydraulic drive according to claim 10 in which said motor is of the type having vanes which are moved inward and outward durin operation of the motor and which includes fluid channels which connect said pilot pump to said motor at the inner edges of said vanes to cause said vanes to be moved outward by liquid delivered to the inner edges thereof by said pilot pump.

WALTER FERRIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,161,439 Thoma June 6, 1939 2,291,011 Vickers July 28, 1942 2,320,727 Herman et a1. June 1, 1943 

